When growing cells of Escherichia coli are exposed to penicillin, cell wall synthesis is inhibited and the cells lyse, releasing their contents into the medium. Our initial observation was that proline requiring mutants were preferentially recovered following mutant isolation using penicillin enrichment. This appears to be due to the absence of free proline in peinicillin treated lysates, although other amino acids are present. The effects of penicillin on proline transport, synthesis and catabolism are being investigated using wildtype strains of E. coli, and mutants defective in proline transport, synthesis and catabolism. This had led to a genetic and biochemical study of proline metabolism and proline transport: the final enzyme in proline biosynthesis (delta 1-pyrroline-5-carboxylic acid reductase) is being purified and characterized; mutants unable to grow on low levels of proline (with defects involving proline transport or utilization) are being isolated and characterized; and mutants with defects in proline degradation are being isolated and characterized.